The present invention relates broadly to video correlation trackers, and more specifically to one which preprocesses video frame data of an electro-optical sensor into histograms based on a predetermined set of gray level values from which binary patterns of a predetermined array of the video frames are formed for use in correlation operations of the tracker for establishing the tracking servo errors with which to govern an optical system which guides the line-of-sight path of the electro-optical sensors.
In a typical video correlation tracker, such as that shown in the block diagram functional schematic of FIG. 1, a conventional gimbaled optical system 20 guides the line-of-sight LOS denoted as a dashed line projected from a target to the field of view of an electro-optical sensor 22, which may be a conventional TV camera, for example. In most cases, the optical system 20 is electromechanically controlled by a set of gimbal servos 24. The TV frame information from the sensor 22 may be provided over signal line 23 to an analog-to-digital converter 26 which is part of the video correlation tracking control electronics 28.
A predetermined portion of the field of view of the sensor 22, more commonly known as the track window, may be partitioned into pixels by the A/D converter 26 and indexed in a pixel memory 30 in accordance with the standard raster scanning of the sensor 22. An example of this pixel partitioning and indexing is shown in FIG. 2A wherein the gray level content of each pixel is denoted as P and the subscripts i, j designate the row, column position in the track window array of pixels. For example, the gray level of the upper left hand corner pixel of the track window of FIG. 2A is denoted as P.sub.0,0 ; and in the bottom left hand corner the gray level of the pixel is denoted as P.sub.31,0. The example of FIG. 2A describes a track window containing a 32.times.32 array of pixels, but it is understood that the track window array may be any prespecified number of pixels.
The conventional tracker A/D converter 26 may additionally convert the gray level pixel data into digital form for storage in a digital storage medium, such as a random access memory RAM, for example. The pixel information of each video frame for the purposes of the typical embodiment of FIG. 1 is stored in the conventional pixel memory 30 for signal processing by a signal processor such as that shown at 32. The signal processor 32 typically determines the position within the track window of a predetermined reference subarray of pixels such as that shown in FIG. 2B, for example. The position of the reference subarray of pixels is normally denoted by the pixel index i, j of the upper left hand corner pixel therein. This reference array is generally established in an acquisition frame and used for correlation operations by a correlator unit 34 in subsequent tracking frames. In most video trackers, the signal processor 32 in conjunction with the correlator unit 34 also determine when the reference subarray of pixels within the track window should be updated so that tracking of the target image will not be lost. Correlation information is normally passed on to a conventional tracker unit 36 wherein typically azimuth and elevation tracking errors may be derived for governing over signal lines 37 the gimbal servos 24 to control the optical system 20 to keep the line-of-sight of the target image within the field of view of the electro-optical sensor 22, for example.
In Applicant's copending application bearing Ser. No. 802,086 filed May 31, 1977 entitled "Apparatus and Method for Preprocessing Video Frame Signals" and assigned to the same assignee as the present application, the signal processor classifies the pixel of the pixel memory for each TV frame as a function of their gray level values. A pixel of an inner array which may be the reference array as shown in FIG. 2B may be grouped as one portion of pixels and the pixels in the annulus surrounding the reference array may be denoted as an outer portion of pixels. Gray level classifications are then performed individually for both the inner and outer portions of pixels for the video information of each TV frame. A statistical discriminate function based on contrasting pixel mass intensity for each of the assigned gray level classifications separates the pixels into modes or groups of consecutive gray level values. Certain modes of gray levels are selected generally as a result of the contrast polarity. Using the measured velocity of the aircraft in which the video correlation tracker is disposed generally derived by an inertial navigation system of the aircraft, the selected gray level modes may often be partitioned in one of either a target set or a background set. Those pixels which are determined to reside in the target set retain their measured video gray level content. All other pixels in the track window are provided with a predetermined gray level value for contrast purposes. The entire digitized gray level code for each of the identified pixels of the target set is used in the correlation operations of the correlator 34 for determining the best match position array of pixels in the subsequent frames of TV information.
In these type video trackers, very complex image matching metric functions are normally used in the correlator 34 while performing auto and cross correlations for providing information to the tracker unit 36 for determining the tracking azimuth and elevation errors and in addition, for determining when an update of the target reference array of pixels may be required. A second copending U.S. application of the Applicant, bearing Ser. No. 944,523, filed Sept. 21, 1978 entitled "An Adaptive Updating Processor For Use In An Area Correlation Video Tracker" and assigned to the same assignee as the present invention, may be used as a reference to illustrate the complexity involved for the auto and cross correlation operations performed in a correlator utilizing the entire digital code of each of the target pixels with respect to the digital codes of the pixels within the track window of the subsequent video frames. Image matching metric functions like the minimum sum of absolute differences, sometimes referred to as MAD, and the compression of partial row and column sum formations are just a few of the many which may be used in the correlations of a conventional video correlation tracker.
It is understood that for some video tracking applications these very precise and complex calculations for the correlation type trackers are necessary to maintain a stabilized image in a scene track. Therefore, it was not hard to justify the amount of hardware and consequently body weight used, and the amount of power consumed by the electronics. Even the amount of derivation calculation time of the correlator in connection with deriving the position of the predetermined reference array of pixels in the tracking frames of video information was warranted. However, for other applications of video correlation trackers such as in remotely powered aircraft and missiles, for example, it has been identified in some cases that the tracking requirements may not be as extreme. In these applications, a video tracker may automatically direct the optical system gimbals thereof to targets on the ground from the air borne craft. In a tracking mode, it may be adequate to have the video tracker merely maintain an image of a prespecified area of the ground within the field of view of the electro-optical sensor which is located on the moving craft.
Apparently, then for purposes of these applications, a reduction in the computational hardware and thus body weight and a reduction in the power consumed would be desirable. In fact, an overall simplication of the correlation procedures so that a more simple video tracker one which would satisfy all the requirements of tracking a ground target from a remotely powered aircraft or vehicle, appears almost necessary to permit visual area correlation trackers to be viable alternative to other types of trackers in competition therewith. Applicant's invention which will be more fully disclosed herebelow is one of a more simplified video correlation tracker for such applications as described hereabove.